Ball Grid Array (BGA) package is widely used which is an encapsulated IC chips mounted on an organic substrate via solder ball array. As the organic substrate, mono- or multi-layers of glass cloth or nonwoven fabric cloth impregnated with an epoxy resin are widely used. In the BGA package, the substrate tends to bend or warp due to difference in volume of encapsulation resin between the topside, where the IC chips are mounted, and the backside of the package. Previously, warpage was mostly concave due to shrinkage associated with curing of the resin. To prevent the concave warpage, a large amount of inorganic filler is added to the resin to suppress the shrinkage, as described in Japanese Patent Application Laid-Open No. 5-267371.
Recently, a volume of an encapsulation resin is decreasing to meet the need for thinner packages. In these packages, the substrate tends to warp convexly, contrary to the aforesaid concave warpage. Particularly, the Chip on Board Ball Grid Array package tends to show convex warpage caused by grooves on the backside to be filled with the resin. To prevent the convex warpage, one may reduce an amount of inorganic filler. However, this may degrade moisture resistance of the encapsulated device. Alternatively, one may use inorganic filler having relatively large coefficient of thermal expansion. A known example of such a filler is spherical cristobalite having an average thermal expansion coefficient of 2.0×10−5/° C. at a temperature of from 40 to 400° C. Compositions comprising the cristobalite are also known from Japanese Patent Application Laid-Open No. 11-302506 and No. 2001-172472. It should be noted that cristobalite exhibits drastic increase in thermal expansion coefficient associated with a phase transition from α-phase to β-phase at 268° C. This causes problems of, for example, separation at an interface between the substrate and the cured resin and a big warpage of the substrate in a reflow process at a maximum temperature of 265° C.